There have been an increasing number of apparatuses such as an electric toothbrush or a cordless telephone, which are operated by cord-less power feeding. In the field of wall-hung television sets and personal computers, there has been development of apparatuses operated by contactless power feeding. Under such circumstances, there have traditionally been suggested and developed a magnetic element for wireless power transmission and a power supply device, each enabling power feeding with high transmission efficiency. For example, Patent document 1 suggests a structure to realize high transmission efficiency and downsizing, which includes a spiral planer coil buried on one side of a magnetic layer, a magnetic window arranged at the center portion of the planer coil, and the magnetic window at least partially structured by a magnetic material.